The invention relates to a solarization-stable borosilicate glass and uses thereof.
Special fluorescent tubes, so-called “backlights”, are used for background illumination of, for example, displays of personal computers, laptops, pocket calculators, vehicle navigation systems, for example.
While conventional fluorescent tubes are made of soft glass, which has a low solarization stability, more solarization-stable glass is needed for backlights whose structure corresponds to that of fluorescent tubes in principle, in order to assure long term functionality.
Because of the structure of the backlights, the glass used must be capable of being melted together with tungsten. To this end it must have a thermal expansion matched to the expansion behavior of W. With the thermal expansion coefficient α20/300 of W of 4.4×10−6/K, glass with α20/300 between 3.7×10−6/K and 4.2×10−6/K is suitable. This is also a difference in respects to the said soft glass, which is melted together with Fe—Ni alloys.
The glass should preferably have a low transformation temperature Tg, i.e. Tg<520° C., so that it can be preferably processed at lower temperatures.
The transmission progression of the glass is essential. As high a possible a transparency is demanded in the visible range in order to obtain a high light yield from the lamp, in the UV range a transmission defined in accordance with the purpose is demanded. For example, the effects of harmful UV radiation ≦260 nm must be prevented by means of a corresponding lowering of the UV transmission in order not to let plastics, for example in laptops, become yellowed and brittle. For this, glass with a UV transmission at lambda ≦260 nm tau of <0.7%, measured at 0.2 mm thick samples, is suitable. For flash tubes or gas discharge lamps, transmissions tau at lambda ≦260 nm of ≦30% (with 0.2 mm thick samples) are sufficient. The transition from the opaque to the transparent wavelength range should be as short as possible, i.e. in this range the transmission curve should extend as steeply as possible.
The minimum demand made on the transmission in the visible wavelength range is, at tau >400 nm and a sample thickness of 0.2 mm, a transmission of 92%. Thus, the requirement is tau (>400 nm; 0.2 mm)≧92%.
A further essential property of glass for “backlights” is the solarization stability which is required for making possible a long service life of the lamps, i.e. as constant as possible a light yield. Glass is to be considered “solarization stable” here which, following 15 hours of HOK-4 radiation, i.e. a radiation from an Hg high-pressure lamp with a main emission at 365 nm and a radiated strength of 850 μW/cm2 at 200 to 280 nm at a distance of 1 m, shows a transmission drop of less than 5% at 300 nm on a glass sample of 0.2 mm thickness. The demands made on glass for flash tubes, gas discharge lamps and backlights are very similar. All should have the highest possible solarization stability and have a high transmission in the visible range.
Various documents are already known in the patent literature describing more or less solarization-stable glass, in particular glass for lamps. However, this glass has the most varied disadvantages, in particular a solarization stability which does not meet the present-day high requirements.
U.S. Pat. No. 5,994,248 describes a headlight lens made of glass of a very broad composition range, part of which is SiO2, and wherein Al2O3, B2O3, earth alkali oxides and alkali oxides, as well as small amounts of iron oxide can be further components. However, the K2O portion is only allowed to lie between 0 and 1 weight-%.
The properties which are essential for backlights, flash tubes and gas discharge lamps, such as solarization stability and a thermal expansion matched to tungsten, are not of importance here.
DE 195 45 422 A1 relates to a bonded glass for anodic bonding of silicon components with glass components, which has a high content of Li2O and contains high Fe2O3/FeO dopings.
JP 10-36135 A describes glass for electronic image capturing. In this case the lowest possible α-radiation of the glass is essential. To this end, in a wide basic glass composition the U, Th, Ra contents are <100 ppb, and the contents of Fe2O3, TiO2, PbO, ZrO are >100 ppm.
EP 0 735 007 B1 describes a solid lead- and arsenic-free glass with resistance against solarization, containing defined amounts of SnO2 and CeO2, by means of which the solarization stability is increased, but not to a satisfactory degree.
The same applies to the glass containing CeO2 and Fe2O3 in WO 98/55413.
The closest prior art is represented by JP 8-12369 A. For UV blocking, the borosilicate glass for gas discharge lamps described therein contains a total of 0.03 to 3 weight-% of at least two of the components V2O5, Fe2O3, TiO2 and CeO2. A high transmission and high solarization stability cannot be accomplished by means of these components with in part large individual proportions, nor by their combination.